Finalization and resurrection

A finalizer (~ClassName in C#) runs cleanup code before an object's memory is reclaimed, usually to release a native resource. Finalization changes the collector's job: an unreachable object with a pending finalizer cannot be freed yet, so the GC must queue it, let a dedicated thread run the finalizer, and only reclaim the memory in a later collection.

Resurrection is the side effect: while queued, the object is reachable again (the queue references it), and its finalizer can even store this somewhere reachable, making an object that was found dead permanently live again. The two flavors of weak reference differ exactly here: a short weak reference (WeakReference by default) is cleared as soon as the object is found unreachable, while a resurrection-tracking one (trackResurrection: true, the WeakTrackResurrection handle type) keeps reporting the object until it is truly gone.

Note

Official docs: Finalizers.

OrionGC keeps the objects awaiting their finalizer in a queue that is a root of every collection, clears the short weak handles before queuing them and the resurrection-tracking ones after, and runs the finalizers after the collection. They run on a dedicated finalizer thread, which Kernel.Start starts when the CosmosEnableFinalizerThread switch is on (the default whenever the scheduler is), and on the thread that collected or allocated otherwise. See Finalization and Handles during marking.